It is known practice to dye keratin fibres, and in particular human keratin fibres such as the hair, with dye compositions containing oxidation dye precursors, which are generally known as oxidation bases, such as ortho- or para-phenylenediamines, ortho- or para-amino-phenols and heterocyclic compounds such as diamino-pyrazole derivatives. These oxidation bases are colourless or weakly coloured compounds which, when combined with oxidizing products, give rise to coloured compounds by a process of oxidative condensation.
It is also known that the shades obtained with these oxidation bases can be varied by combining them with couplers or coloration modifiers, the latter being chosen in particular from aromatic meta-diamines, meta-aminophenols, meta-diphenols and certain heterocyclic compounds such as indole compounds.
Another advantage of this type of dyeing is that it is visible on dark hair. Effectively, since the oxidative process is performed in an alkaline medium, most generally in the presence of aqueous ammonia, the bleaching of melanin, the natural pigment of keratin fibres and especially of the hair, takes place in parallel with the condensation of the dye precursors. The colour obtained may thus also be visible even on dark hair.
The colorations obtained show good fastness in particular with respect to shampooing. However, it is rare to be able to obtain chromatic colorations via this method.
It is also known practice to dye keratin fibres with compositions containing direct dyes. These compounds are coloured and colouring molecules with affinity for keratin fibres. They are applied to the fibres for a period of time necessary to obtain the desired coloration, and are then generally rinsed out.
The standard direct dyes used are in particular dyes of the nitrobenzene, anthraquinone, nitropyridine, azo, cationic azo, xanthene, acridine, azine or triaryl-methane type or natural dyes.
It is possible to obtain colorations that are lighter than those of the hair before dyeing, if the direct dyeing is carried out in the presence of an oxidizing agent. This is then referred to as direct dyeing under lightening conditions.
Direct dyeing advantageously makes it possible to achieve very chromatic colours, but they always have the drawback of being temporary or semi-permanent. In fact, the fastness of direct dyes on the hair remains limited, which leads to fading of the colour, or even to the colour changing over time, due to loss of one or more of the dyes used.
One drawback of these two dyeing modes is the need to use an oxidizing composition for the oxidation dyeing or to obtain lightened direct dyeing. It is known that oxidizing compositions, in the long run, cause degradation of hair fibres.
Another drawback of these two dyeing modes lies in the fact that the compositions are initially coloured, as in the case of dye compositions using direct dyes, or else they become coloured during the application, as is the case of compositions comprising one or more oxidation dye precursors. Consequently, direct dyeing and oxidation dyeing have the drawback of being soiling.
French Application FR 05 52277 has recently described the use of compounds of styryl or imine type existing in a coloured form and in a colourless form, for dyeing keratin fibres. Thus, under certain conditions, the composition used is colourless or weakly coloured and the colour is revealed in the keratin fibres once the composition has been applied, which makes it possible to solve the problem of staining of the skin and of the fabrics used.
Although these compositions afford many advantages over the existing compositions, while at the same time offering satisfactory dyeing efficacy, it nevertheless. remains that it may be desired to further increase the uptake and to reduce the selectivity of the coloration obtained.